Diagnosis. Shell conical, of limpet form, with anterior slit, notch, or dorsal foramen marking position of exhalent siphon; operculum lacking in adult. Shell composed of lamellar aragonite (some with exterior calcitic layer); shell interior not nacreous. Sculpture of radial ribs and concentric ribs of lesser strength; muscle scar horseshoe-shaped. Heart diotocardian, ctenidia paired, of equal size, with afferent and efferent membranes. Left kidney reduced, nearly vestigial. Epipodial tentacles stubby, in single row along sides of foot. Mantle folds capable of expansion to envelop shell, head, and foot. Radula rhipidoglossate, strongly asymmetric, the rachidian and 4 laterals weakly cusped, pluricuspid (outermost fifth lateral) large and strongly cusped; marginals numerous.
Biology. Members of the geologically oldest genera live from shallow to moderate depths and are known to graze upon encrusting invertebrates, particularly sponges and compound ascidians. The youngest genus, Fissurella Bruguiere, 1789, feeds upon intertidal algae. Fissurellids are broadcast spawners; the planktonic veliger stage is short (Fretter and Graham, 1976).
Remarks. The fissurellid limpets are known as the slit limpets or the keyhole limpets, owing to the slit or fissure in the shell that serves as the excurrent opening from the mantle cavity. This is the only limpet group with paired gills.
The subfamily classification followed here is modified from that of Thiele (1929). Two subfamilies, the Emarginulinae and the Fissurellinae, are recognized, based primarily on radular differences (McLean, 1984, both references). Characters of the radula and the ontogeny of the slit or foramen provide the basis for the principal dichotomy. In the older Emarginulinae, the radula has a rhomboidal rachidian tooth, the outer lateral tooth is usually bicuspid, there are varying expressions of the slit or foramen, and a selenizone is represented, if only in early shell ontogeny. In the Fissurellinae, the rachidian tooth has a narrow neck and is similar in shape to the adjacent laterals; the outer lateral tooth is usually quadricuspid. The excurrent opening in the shell is always an apical foramen and there is no indication of a selenizone in early ontogeny.